This invention relates to application of a sliding mode control process to an engine air-fuel ratio control.
U.S. Pat. No. 6,266,605 discloses an application of a sliding mode control process to the feedback control of the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine.
In this prior art technique, the engine as a controlled plant is represented by a continuous physical model and a state equation is obtained from the physical model. Specifically, according to this prior art, a state equation of a continuous model is first obtained from the physical model and this equation is then converted into a state equation of a discrete system.
However, in the process of obtaining the discrete state equation from the continuous physical model, it is difficult to set a physical model that precisely represents the controlled plant. Especially, it is difficult to set a physical model when the controlled plant has a strong nonlinear characteristics like an internal combustion engine. Due to this reason, the prior art sliding mode control process approximated the response delay of the controlled plant as a second order delay in a continuous system.
It is therefore an object of this invention to provide a more precise physical model for the sliding mode control of an engine air-fuel ratio.
In order to achieve the above object, this invention provides an air-fuel ratio control device for an engine with a combustion chamber for burning a mixture of air and fuel. The device comprises a mechanism which regulates an air-fuel ratio of the mixture, an air-fuel ratio sensor which detects the air-fuel ratio of the mixture from a composition of an exhaust gas of the engine; and a programmable controller.
The controller is programmed to calculate a state quantity "sgr"(n) of a switching function based on a preset target air-fuel ratio TGABF, a detected air-fuel ratio AFSAF detected by the sensor, a state equation derived from a transfer function Geng(q) of a secondary discrete system representing the correlation between the air-fuel ratio of the mixture in the combustion chamber and the detected air-fuel ratio, and a predetermined switching function gain mat S, calculate a non-linear input unl(n) for a sliding mode control process based on the state quantity "sgr"(n) of the switching function, the predetermined switching function gain mat S, a predetermined non-linear gain xcex7 and a predetermined disturbance input xcex2(n), calculate a linear input ueq(n) for the sliding mode control process based on a difference between the target air-fuel ratio TGABF and the detected air-fuel ratio AFSAF, and the switching function gain mat S, calculate an air-fuel ratio feedback correction value ALPHA from the non-linear input unl(n) and the linear input ueq(n), and feedback control the air-fuel ratio regulating mechanism based on the air-fuel ratio feedback correction value ALPHA.
This invention also provides an air-fuel ratio control method for such an engine that is provided with a combustion chamber for burning a mixture of air and fuel, and a mechanism which regulates an air-fuel ratio of the mixture. The method comprises detecting the air-fuel ratio of the mixture from a composition of an exhaust gas of the engine, calculating a state quantity "sgr"(n) of a switching function based on a preset target air-fuel ratio TGABF, a detected air-fuel ratio AFSAF, a state equation derived from a transfer function Geng(q) of a secondary discrete system representing the correlation between the air-fuel ratio of the mixture in the combustion chamber and the detected air-fuel ratio, and a predetermined switching function gain mat S, calculating a non-linear input unl(n) for a sliding mode control process based on the state quantity "sgr"(n) of the switching function, the predetermined switching function gain mat S, a predetermined non-linear gain xcex7 and a predetermined disturbance input xcex2(n), calculating a linear input ueq(n) for the sliding mode control process based on a difference between the target air-fuel ratio TGABF and the detected air-fuel ratio AFSAF, and the switching function gain mat S, calculating an air-fuel ratio feedback correction value ALPHA from the non-linear input unl(n) and the linear input ueq(n), and feedback controlling the air-fuel ratio regulating means based on the air-fuel ratio feedback correction value ALPHA.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.